Scientists have discovered the oldest “victim” of the plague that eventually went on to cause a devastating global pandemic in the 14th century. Termed the “Black Death,” the plague killed almost a third of the European population — besides striking countries like China, Egypt, and India at different points in time.

The newly-discovered “patient zero” was a male hunter-gatherer, who died more than 5,000 years ago in Latvia while he was between 20 to 30 years old. He is was infected with the earliest-known strain of the plague-causing bacteria Yersinia pestis, which ravaged parts of Europe and Asia 2,000 years after infecting this man.

“Up to now this is the oldest-identified plague victim we have,” Ben Krause-Kyora from the Institute of Molecular Biology at the Christian-Albrecht University of Kiel in Germany, who co-authored the study, told BBC News.

The researchers found the hunter-gatherer buried with three other individuals — a woman, an infant, and another man — at a burial site in Latvia. As part of the study, the researchers sequenced DNA from their bones and teeth to test for bacterial and viral pathogens.

“He most likely was bitten by a rodent, got the primary infection of Yersinia pestis, and died a couple of days [later] — maybe a week later — from the septic shock,” Krause-Kyora added.

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Published in Cell Reports, the study found that the strain of the bacteria was different from those found later in the Neolithic and bronze ages. This strain lacked the gene that allowed future versions of it to be spread by fleas. Researchers believe it was not only weaker than the future strains — but also incapable of spreading rapidly.

“Even a small shift in genetic settings can have a dramatic influence on virulence… We think that these early forms of Yersinia pestis couldn’t really drive big outbreaks,” Krause-Kyora said.

However, the findings challenge the theory about disease and population in the 14th century: researchers have previously postulated it was an erstwhile iteration of the Yersinia pestis bacteria that was responsible for the drastic decline in the European population around the time the new “patient zero” died.

Krause-Kyora believes that the new discovery — coupled with the fact that the “patient zero” seemed to have been buried with care and that researchers are yet to discover “plague pits” from the era — suggests that Yersinia pestis may not have driven population-declines 5,000 years ago.

Other scientists are of the opinion that this study doesn’t conclusively absolve Yersinia pestis of all blame. “We know that large settlements, trade, and movement happened in this period and human interaction is therefore still a very plausible cause of the spread of plague in Europe at this time,” said Simon Rasmussen, a biologist from the University of Copenhagen, who has studied historic strains of the bacteria, but wasn’t involved in the present study.

While it may take a while to settle the debate about the severity of the earliest strain of Yersinia pestis, scientists are hopeful the discovery will help fill some gaps in existing research on the evolution and spread of the pathogen. “What’s most astonishing is that we can push back the appearance of Yersinia pestis 2,000 years farther than previously published studies suggested,” Krause-Kyora noted.

“It seems that we are really close to the origin of the bacteria.”